Seeing the light? Study illuminates how quantum magnets mimic light

Crystal of the quantum spin ice candidate Pr2Hf2O7 used in the study. Credit: Romain Sibille

What is light? It sounds like a simple question, but it is one that has occupied some of the best scientific minds for centuries.

Now, a collaborative study with scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) has added another twist to the story, turning an abstract theory about the quantum properties of magnets into a testable hypothesis about a new kind of light.

Ever since Isaac Newton refracted light through prisms in 1672, scientists have been split over whether light is made up of particles or waves. Light seems to travel in straight lines, as would be expected of a particle, but Newton's experiments have shown it also has frequency and wavelength, like sound waves.

Almost 200 years later, the Scottish physicist James Clerk Maxwell supplied one part of the answer, when he realized that light was made up of fluctuating electric and magnetic fields. It was only in the 20thcentury through the work of Einstein, that light was finally understood to be made up of fundamental particles called photons, which act like both particles and waves.

This discovery helped inspire the new science of quantum mechanics, which describes the behavior of matter and energy on the atomic and subatomic level.

More recently, in the late 20th century, physicists began exploring a phenomenon called emergence. Just as the behavior of large groups of people can differ from that of any single member of the group, emergence describes how particles in large groups can behave in unexpected ways, revealing new laws of physics or providing a new context for old ones. One question being asked was, "Could there be such a thing as emergent light?"

This brings us to OIST Professor Nic Shannon, Han Yan, a Ph.D. student in his Theory of Quantum Matter Unit, and their colleagues in Switzerland and in the US. Their recent work centers on a strange family of magnetic systems known as spin ice, which escape all conventional forms of magnetic order and instead open a window on the quantum world.

In conventional magnets like the ones on your fridge, magnetic atoms produce a tiny magnetic field and work together to generate the much larger magnetic fields which enable them to "stick" to metal objects. This is possible because the tiny magnetic fields associated with each different atom in the magnet order themselves so that they point in the same direction.

In spin ice, however, atoms do not order magnetically, but still work together to produce a magnetic field which fluctuates on the atomic scale.

Recently, researchers realized that quantum effects at low temperatures can introduce an emergent electric field in spin ice, with an amazing consequence: Emergent electric and magnetic fields combine to produce magnetic excitations that behave exactly like photons of light.

"It behaves like light, but you can't see it with your eyes," said Prof Shannon "Imagine the crystal of spin ice is a tiny universe with its own laws of nature, and you are on the outside looking in. How could you figure out what is going on inside.

In 2012 Prof. Shannon and his then Ph.D. student Owen Benton proposed a way to detect the light inside a quantum spin ice by bouncing neutrons off the magnetic atoms inside the crystal. They predicted a characteristic signature in how the crystal absorbs the energy of the neutrons, which signals the presence of the emergent electrodynamics of a quantum spin ice.

Now, in a paper published in Nature Physics, the authors report that they have observed this signature in a material called praseodymium hafnate (Pr2Hf2O7).

Finding the signatures of emergent light in a real material proved very challenging, as it required working at temperatures as low as 50 milikelvin—less than a tenth of a degree above absolute zero—with crystals free of any dirt and imperfections.

A research team lead by Dr. Romain Sibille from the Paul Scherrer Institut (PSI) in Switzerland, in collaboration with colleagues at the University of Warwick in the UK, managed to generate a perfect crystal of a quantum spin ice material with which they could finally test the hypothesis.

"It's very beautiful, like a precious stone," said Prof Shannon, "and it's amazing to think it's all one big crystal with no imperfections."

Sibille took this crystal to the European Institut Laue-Langevin (ILL) in Grenoble, France, as well as the Oak Ridge National Laboratory (ORNL) in Tennessee, USA, to use these facilities' specially-developed neutron spectrometers.

In an extremely challenging experiment, Sibille's team used an array of 960 supermirrors coated with iron, cobalt, and vanadium alloys that could selectively reflect different types of neutrons—something that his home institution PSI has developed, and used the HYSPEC instrument (ORNL) to obtain a 3-D analysis of their reflection patterns.

In combination with a thorough mapping of the scattered neutrons using the IN5 instrument (ILL), this allowed them to measure the polarization of the scattered particles and map the energy signatures those particles produced."

Dr. Benton and Prof Shannon's theory bore an uncanny similarity to the experimental energy maps. The graphical representation of neutron reflection displayed so-called pinch points, which are characteristic features of a quantum spin ice. When the spin ice was scanned at low temperatures, the pinch points disappeared in a way that strongly suggested emergent light.

Yan worked on the theory and analyzed the experimental data to determine the speed of the emergent light—a modest 3.6m per second, about as fast someone running a marathon in four hours. The photons of normal light—the kind you might sunbathe under—could cover the same distance in less than a thousandth of a second.

"To me it's very cool that this material behaves like a mini-universe with its own light and charged particles" said Han.

"At present, we don't know any way of explaining these results without invoking quantum mechanics," said Prof Shannon, "so it really does look like we have seen emergent light."

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User comments

An aspect of Maxwell's model of light no one seems to even consider. Maxwell modeled light as, first of all, mutual electric and magnetic fields not connected to any matter, effectively disembodied. Most fields are thought of as emanating from a material particle in some way. There are alternating electric and magnetic fields traveling on their own through space. What fives rise to them? The alternating electric field gives rise to the alternating magnetic field, but the alternating magnetic field is giving rise to the alternating electric field, but the alternating electric field that the alternating magnetic field gives rise to gives rise to the alternating magnetic field that causes the alternating electric field. It can be said that Maxwell envisioned light's fields as giving rise to themselves.

When the center of a charge's field moves all equal potential spheres are updated, when this center moves, a magnetic field is updated, light! or Photon! but note charge at steady state then oscillate it, see the fields update. Light! Wheeeeee!

When the center of a charge's field moves all equal potential spheres are updated, when this center moves, a magnetic field is updated, light! or Photon! but note charge at steady state then oscillate it, see the fields update. Light! Wheeeeee!

When the center of a charge's field moves all equal potential spheres are updated, when this center moves, a magnetic field is updated, light! or Photon! but note charge at steady state then oscillate it, see the fields update. Light! Wheeeeee!

Every theory, story, and even experiments, if it does not know what is matter, from which it forms and forms, are all in vain attempts to reach the true causes of the phenomenon.The substance is formed from the substance Aether, which fills the infinite universe.One kind of matter, and these are gluons, together with Aether, cause the appearance of magnetism, which is the basis of all waves and spin particles in motion.Learn All:Neutrons are magnetizers, especially when the chemical element has more neutrons than proton. Tell those scientists to show them how they can handle it easily. What bothers them in vain, when they do not understand how they are formed: matter, gravity, and magnetism.

Every theory, story, and even experiments, if it does not know what is matter, from which it forms and forms, are all in vain attempts to reach the true causes of the phenomenon.The substance is formed from the substance Aether, which fills the infinite universe.One kind of matter, and these are gluons, together with Aether, cause the appearance of magnetism, which is the basis of all waves and spin particles in motion.Learn All:Neutrons are magnetizers, especially when the chemical element has more neutrons than proton. Tell those scientists to show them how they can handle it easily. What bothers them in vain, when they do not understand how they are formed: matter, gravity, and magnetism.

@Hyperfuzzy,You need help, because you show that you do not know anything from what I say. You do not understand what matter is and what it forms, and I also see that you do not know anything about the properties and the origin of gravity and magnetism. I do not need help, but I am trying to get you back to the state to understand, first of all, who you are, who gave you the skills or you have awareness, or that you do not have it at all.How do you not feel embarrassed to make such statements and evaluations for those people who want to help you understand the phenomena that the entire science has not realized. You'll soon see how contaminated with stupid theories.

@Hyperfuzzy,You need help, because you show that you do not know anything from what I say. You do not understand what matter is and what it forms, and I also see that you do not know anything about the properties and the origin of gravity and magnetism. I do not need help, but I am trying to get you back to the state to understand, first of all, who you are, who gave you the skills or you have awareness, or that you do not have it at all.How do you not feel embarrassed to make such statements and evaluations for those people who want to help you understand the phenomena that the entire science has not realized. You'll soon see how contaminated with stupid theories.

Does this cause you to feel more confident in your nonsense; by disrespecting others without reason or logic?

@Hyperfuzzy, I'm thinking, does it make sense with such people to conduct any discussion? You look like a balloon filled with negative energy, which you use to disparage those you do not even understand. The reason is that your awareness is on exhalation. When I give you some explanations that oppose the view and knowledge of today's tycoon of science from this avenue, then you direct most of your negative energy to destroy what is true and which can not be destroyed. You are not able to understand what I am proving, because you are irreversibly contaminated by stupid theories.The first step for your education is: you have to understand that there must be a substance in the universe from which matter is formed. If you ignore it, there is no way to learn anything in the universe and learn something. TRY !!

@Hyperfuzzy, I'm thinking, does it make sense with such people to conduct any discussion? You look like a balloon filled with negative energy, which you use to disparage those you do not even understand. The reason is that your awareness is on exhalation. When I give you some explanations that oppose the view and knowledge of today's tycoon of science from this avenue, then you direct most of your negative energy to destroy what is true and which can not be destroyed. You are not able to understand what I am proving, because you are irreversibly contaminated by stupid theories.The first step for your education is: you have to understand that there must be a substance in the universe from which matter is formed. If you ignore it, there is no way to learn anything in the universe and learn something. TRY !!

almost correct, there is only charge that i see from whatever you said, with no disrespect; try it, use an nxn set of equation for 2m centers, reconcile the rest using

try logic1st reconcile QM is Not Science; didn't I reconcile the actuality with causality, what is light 'is'using known and accepted science that is also used to design our instrumentation! So explain my science, Coulomb, Maxwell, discovery that a neutron is composed of an electron, a proton and a neutrino, or a time varying created during separation = neutrino

College kids, good at math; basics? What is charge? Isn't that the first question? QM = rubbish, taught, not defined! Now we have vast computational power and 1 simple object at every "Active" point in space, for each point is simply the summation over a volume off space-time.

try my unit less isomorphic space where each point is described by an object, 1. 'what' at that point subtracted from the summation of all fields, relative to that point, etc. history, ... set c =1, time and space same magnitude, no units, q = +/- 1; rest, show whatever

College kids, good at math; basics? What is charge? Isn't that the first question? QM = rubbish, taught, not defined! Now we have vast computational power and 1 simple object at every "Active" point in space, for each point is simply the summation over a volume off space-time.

try my unit less isomorphic space where each point is described by an object, 1. 'what' at that point subtracted from the summation of all fields, relative to that point, etc. history, ... set c =1, time and space same magnitude, no units, q = +/- 1; rest, show whatever

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